Method of and apparatus for the reduction of thermionic emission in discharge devices



Feb. 5, 1963 J. TURNER ET AL 3,076,911

METHOD OF AND APPARATUS FOR THE REDUCTION OF THERMIONIC EMISSION IN DISCHARGE DEVICES Filed May 18, 1960 I lNV NTORS METHQD fil AND APPARATUS FUR THE REZJUQ- TEEN (BF THERll/ilQI llC EMESSHQN 1N Did- CHARGE DEVli'JEEa .lohn L. Turner, Needhnin, and Seymour Goldberg, Lexington, Mass, asslgnors to Edgerton, Germeshausen & Grier, Inca, Boston, Mass, a corporation or Massachusetts Filed May 18, H59, Ser. No. 2994s 14- Ciaims. (6i. Slit-1&7)

The present invention relates to electric-discharge devices and, more particularly, to discharge tubes and the the involving electron or similar emissive electrodes and other surfaces.

Emission in thermionic tubes from elements other than the cathode is a serious problem. It can, for example, it the emission is from the control grid, cause a leakage current in the control rid circuit, thereby detrimentally lowering the grid input impedance. This problem of spurious emission from the grid electrode becomes even more serious in the case of Witching tubes that require high anode hold-off voltage, such as hydrogen thyratrons of the type disclosed in United States Letters Patent No. 2,842,699, issued July 8, 1958, to Kenneth J. Germeshausen and the applicant herein, Seymour Goldberg, because a substantial amount of the grid-emission leaka e current flows to the anode and When this current is multiplied by the high anode voltage, it represents a source of considerable anode dissipation. Typical switching tube applications include a radar modulator, a pulsegenerator, 21 stroboscope or a flash-photography system or the like. In gas-iilled tubes, moreover, the ionization caused by the how of the grid-emission current to the anode can often readily cause the grid to lose control of the tube and cfiect premature anode firing.

Such undesirable spurious emission in thermionic tubes is caused by evaporation or sputtering of active emitting materials from the cathode upon the various elements or electrodes, such as the control grid. When such an element or electrode becomes heated, either by radiation from the cathode or by some other power dissipated in the tube, the electrons will be spuriously thermionically emitted from that element or electrode and will flow to the anode.

Various attempts have been made throughout the years to solve this problem of undesirable emission in thermionic tubes. One proposal has involved coating the element or electrode, from which it is desired to prevent emission, with an electron-emission suppressing material, or fabricating the element or electrode from such a material. This type of solution, however, has often presented many new problems as a result of the characteristic differences between the suppressant material and the tube elements or electrodes. Gold, for example, has been a popular electron emission suppressant; but gold has the disadvantages that it has a high vapor pressure, that there is loss of diffusion into the base metal, and that the cathode often becomes accordingly poisoned. Other disadvancages in the prior use of emission-suppressing materials upon the electrode from which spurious emission is to be suppressed, include the fact that thin suppressant layers are often sputtered oil by ion bombardment. After long periods operation of tubes having, for example, a grid electrode containing thereupon an electron-emission suppressant, moreover, the grid becomes covered with a suiliciently thick layer of active cathode materials and/ or other materials sputtered from other members of the tube, to render the emission suppressant on the base metal ineffective.

In accordance with the present invention, on the other hand, it has been discovered that if appropriate electron- Zifi'lhfi Patented Feb. 5, 1%

emission suppressant materials are not applied to the particular electrode from which it is desired to prevent spurious emission but rather are appropriately disposed with respect to this electrode, the desired result may be achieved Without any of the disadvantageous features before discussed. An object of the invention, therefore, is to provide a new and improved method of and apparatus for reducing electron emission from other elements or electrodes in the tube than the cathode, and that shall not be subject to the disadvantageous features of the prior art.

A further object is to provide a new and improved electron-emission suppressing structure.

Other and further objects will be explained hereinafter and will be more particularly pointed out in connect on with the appended claims.

The invention will now be described in connection with the accompanying drawing the single of which is a longitudinal section of a gaseous discharge tube constrncted in accordance with a preierred embodiment of the invention.

While the invention is shown applied to a particular type of gaseous-discharge device, it will be evident that the invention is equally adaptable to other types of discharge devices and it is, accordingly, not to be construed as limit: to the illustrated device. The tube shown in the drawing is of the type disclosed in the said Letters Patent, embodying a cup-shaped anode 9 having an active substantially planar surface 17 and a similar backing-plate surface 19 disposed on opposite surfaces of ti e planar bottom wall of the anode cup 9. The anode serves as an upper Wall of the ceramic or similar envelope 3, thereby to dissipate heat. The anode cup 9 may be of copper and the active planar electrode surface 17 may be of molybdenum or the like, with the backing late ill of Kovar material or the like. Flanges 13 are shown extending outward of the envelope 3 between a ceramic backing ring 3 and the main ceramic cylindrical Wall 3 of the tube, being sealed thcrebetween as taught, for example, in the said Letters Patent. The external portions or" the flanges 13 may be curved into a rim l3 and may serve not only to dissipate heat, but to reduce potential gradients in the region of the seal and to establish electrical connection to the anode 9.

An inverted heat-dissipating cup-shaped control electrode or grid 37, as of copper and the like, is disposed with its upper substantially planar surfaces apertured at 41, in close proximity to and substantially parallel to the active surface 17 of the anode 9. Depending from the central region of the planar surface 3% of the grid 37, is a reinforcing mem er 2t) having grooves 22 intermediate the apertures ll in the planar region 35? and so positioned to provide an electron path through the reinforcing member 26' and the grid 39 to the anode surface 17'. The control electrode or grid 37 is provided with externally extending feet or flange l3 sealed between the bottom of the cylindrical envelope wall 3 and a lower extension 3 thereof, as in the manner discussed in the said Letters Patent.

The cathode structure, indicated generally by 2.1, is disposed within the grid cup 37-39. Electrical heater and voltage connection to, and support for, the cathode structure Zll may be effected by plugs 27, sealed through the ceramic base 5 of the envelope. A gas intubulation chamber 27 may similarly be sealed into the base 5. The cathode 21 is shown comprising a set of substantially parallel planar vanes 21a extending from a common base 215, which may be heated as described in copending application Serial No, 589,551, filed April 25, 1956, by the said Kenneth J. Germeshauscn and Seymour Goldberg for Electric-Discharge Device and Cathode, now

aorscn Us. Letters Patent No. 2,937,301, issued May 17, 1960, or heated through the passage of current therethrough, as disclosed in application Serial No. 660,592, filed May 21, 1957, by the said Seymour Goldberg, for Electric- Discharge Device and Cathode, now US. Letters Patent No. 2,937,302, issued May 17, 1960. As disclosed in said applications, the vane surfaces 21a are coated with electron-emissive material, as is well known, in order to enable the emission of substantial quantities of electrons upward through the apertures 22 of the reinforcing member 20, and 41 of the control grid electrode 39-37 to the active surface 17 of the anode 9.

The cathode Z1 is shown contained within a cup-like structure 210 which, in turn, is supported in a larger cylindrical structure 21d terminating in outwardly extending heat-conducting and electrical-connection-providing flange 21c, sealed, as before described, between the envelope portion 3 and the base 5. Gaseous reservoir 21, may be employed, such as that described in United States Letters Patent No. 2,919,368, issued December 29, 1959, to the said Seymour Goldberg and Edward I. Goon, for Gaseous Reservoir and Method, having within the container 21f, for example, a rare earth or the like, carrying absorbed gas so as to diffuse the gas into the tube to replace gas that has been used up during operation. interposed in the space between the electron-emissive cathode vanes 21a and the control-grid electrode surface 39 is an open-sided cover 21g for the cathode cup 210. The cover 21g is provided with an aperture 21h above which is suspended a baffle piece Zli.

In accordance with the present invention, it has been discovered that if the elements 21g and iii, interposed between the electron-cmissive surfaces 21a of the cathode and the grid surface 39 from which it is desired to prevent electron emission, is coated with the electron-emission suppressing material, rather than coating that electronemission suppressing material on the surface 39 itself from which it is desired to suppress electron emission, the desired result can be obtained without any of the disadvantages of the prior art, before stated.

As an illustration, a thin foil or layer of platinum was applied to the upper and lower surfaces of the elements 21g and 151i indicated in the drawings by the letters A, B, C and D, interposed between the region of electron emission from the active portions 21a of the cathode and the surface 39 of the electrode 37 from which it is desired to prevent electron emission, This was found to reduce electron emission from the grid 37-39 in the tube by a factor of one hundred, and without the disadvantageou features before referred to that occur when the suppressing material is directly coated upon the grid itself.

The invention, however, is not limited to platinum foil, but may be practiced with similar results by using other similar suppressants such as, for example, titanium, niobium, gold, and the like. The suppressants, moreover, need not be applied as foil coverings, but may be provided by other techniques including electro-plating, sintering, and the like. In fact, the element itself may be constructed of the electron-emission suppressing material and thereby preclude the use of a coating.

By describing the invention in terms of the preferred embodiment, it is by no means limited to this particular embodiment. e invention has utility whenever the electron-ernission suppressing material is so positioned within the discharge device that a substantial portion of the electron emission from the cathode during discharge comes in contact with or in close proximity to the suppressant before the discharge reaches the region of the element from which it is desired to prevent spurious emission.

While it is not fully understood how the reduction in the grid emission is obtained by placing the platinum or other foil or covering on the cathode baffle structure 21g and 211', itbeing sufficient to describe the invention as it has been found to work in practice, providing very large improvement in results, it is believed that one of the following theories may provide a possible explanation.

Under discharge conditions, the cathode baffie elements 21g and Zlli are bombarded with high-energy gas ions, causing sputtering and continuous relocation of the emission-suppressant on the grid 39. This, then, prevents a thick layer being formed that would render the emission suppressant ineffective, as occurs when the suppressant is applied directly to the grid or other electrode or eleent from which emission is to be suppressed. Another possible explanation may reside in the fact that a large part of the emissive material from the cathode vanes 21a which is eventually deposited on the grid surface 39, is first deposited on the battle elements Zlg-Zli interposed therebetween and is therefore rendered inoccuous through a process of mixing with the suppressant at A, B, C and D.

Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention, as defined in the appended claims.

What is claimed is:

1. An electric-discharge device having, in combina tion, an electron-emissive cathode electrode and at least an additional electrode and the like displaced from the cathode wihin the discharge device, and toward which electrons may be emitted from the cathode, the addition a1 electrode being adapted to produce spurious electron emission upon the sputtering thereupon of electronemitting material from the cathode, and means for inter-- posing a surface that inherently suppresses electron ends-- sion in the region of the electron path between the electron-emissive cathode and the said additional electrode to inhibit the said spurious emission from the additional electrode.

2. An electric-discharge device having, in combination, an electron-emissive cathode electrode and at least an additional electrode and the like displaced from the cathode within the discharge device, and toward which electrons may be emitted from the cathode, the additional electrode being adapted to produce spurious electron emission upon the sputtering thereupon of electron-emitting material from the cathode, a surface of an electronemission-suppressing material, said surface being disposed in the region of the electron path between the electronemissive cathode and the said additional electrode to inhibit the said spurious emission from the additional electrode.

3. An electric-discharge device having, in combination, on electron-emissive cathode electrode and at least an additional electrode and the like displaced from the cathode within the discharge device, and toward which elec-' trons may be emitted from the cathode, the additional electrode being adapted to produce spurious electron emission upon the sputtering thereupon of electron-emitting material from the cathode, and a further element disposed in the region between the electron-emissive cathode and the said additional electrode in the path of the emitted electrons, said further element being composed of an electron-emission-suppressing material to inhibit the said spurious emission from the additional electrode.

4. An electric-discharge device as claimed in claim 3 and in which the said electron-emission-suppressing material is selected from the group comprising platinum, titanium, niobium, and gold.

5. An electric-discharge device having, in combination, an electron-emissive cathode electrode and at least an additional electrode and the like displaced from the cathode within the discharge device, and toward which electrons may be emitted from the cathode, the additional electrode being adapted to produce spurious electron emission upon the sputtering thereupon of electron-emitting material from the cathode, a further element disposed in the region between the electron-'emis'sive cathode and the said additional electrode in the path of the emitted electrons, and an electron-emission-suppressing material disposed upon the further element to inhibit the said spurious emission from the additional electrode.

6. Apparatus as claimed in claim 5 and in which the further element comprises an apertured cover having bafile means.

7. Apparatus as claimed in claim 6 and in which the said cover and bafiie means is coated with the e1ectronemission-suppressing material and is electrically connected to the electronemitting cathode.

8. An electric-discharge device as claimed in claim 7 and in which the said electron-emission-suppressing ma \terial is selected from the group comprising platinum, titanium, niobium and gold.

9. An electric-discharge device having, in combination, an electron-emissive cathode electrode and at least an additional electrode and the like displaced from the cathode within the discharge device, and toward which electrons may be emitted from the cathode, the additional electrode being adapted to produce spurious electron emission upon the sputtering thereupon of electron-emitting material from the cathode, a further element disposed in the region between the electron-emissive cathode and the said additional electrode in the path of the emitted electrons, and an electron-emission-suppressing material disposed as a coating upon portions of the further element to inhibit the said spurious emission from the additional electrode.

10. An electric-discharge device having, in combination, an electron-emissive cathode electrode and at least an additional electrode and the like displaced from the cathode within the discharge device, and toward which electrons may be emitted from the cathode, the additi'on-al electrode being adapted to produce spurious electron emission upon the sputtering thereupon of electronemitting material from the cathode, a conductive element disposed in the region between the electron-emissive cathode and the said additional electrode in the path of the emitted electrons, and an electron-emission-suppressing material disposed as a coating upon portions of the conductive element to inhibit the said spurious emission from the additional electrode.

11. An electric-discharge device as claimed in claim 10 and in which the said electron-emission-suppressing material is selected from the group comprising platinum, titanium, niobium and gold.

12. An electric-discharge device having, in combination, an electron-emissive cathode electrode and at least an additional electrode and the like displaced from the cathode within the discharge device, and toward which electrons may be emitted from the cathode, the additional electrode being adapted to produce spurious electron emission upon the sputtering thereupon of electronemitting material from the cathode, a conductive element disposed in the region between the electron-emissive cathode and the said additional electrode in the path of the emitted electrons, and an electron-emission-suppressing material disposed upon the conductive element to inhibit the said spurious emission from the additional electrode.

13. An electric-discharge device as claimed in claim 12 and in which the conductive element is connected electrically with the cathode.

14. An electric-discharge device as claimed in claim 13 and in which the said electron-emission'suppressing material is selected from the group comprising platinum, titanium, niobium and gold.

References Cited in the file of this patent UNITED STATES PATENTS 2,399,003 Crapuchettes Apr. 23, 1946 2,417,460 Eitel et al. Mar. 18, 1947 2,516,841 Arditis et al. Aug. 1, 1950 2,518,879 Germeshausen Aug. 15, 1950 2,831,999 Doolittle Apr. 22, 1958 2,919,368 Goldberg et al. Dec. 29, 1959 

1. AN ELECTRIC-DISCHARGE DEVICE HAVING, IN COMBINATION, AN ELECTRON-EMISSIVE CATHODE ELECTRODE AND AT LEAST AN ADDITIONAL ELECTRODE AND THE LIKE DISPLACED FROM THE CATHODE WITHIN THE DISCHARGE DEVICE, AND TOWARD WHICH ELECTRONS MAY BE EMITTED FROM THE CATHODE, THE ADDITIONAL ELECTRODE BEING ADAPTED TO PRODUCE SPURIOUS ELECTRON EMISSION UPON THE SPUTTERING THEREUPON OF ELECTRONEMITTING MATERIAL FROM THE CATHODE, AND MEANS FOR INTER- 